1. Technical Field of the Invention
The present invention relates to a cellular telecommunication network, and more particularly, to an improved system and method for implementing a backup to the network's control channel.
2. Description of Related Art
In modern cellular telecommunication networks, the networks are divided into cells which service a defined area for mobile stations. Each cell contains a base station which transmits and receives voice and control information to and from mobile stations (mobile phones) which are located within the cell's coverage area. Each of the cells employs a separate control channel for relaying control information to mobile stations, and voice channels for relaying voice information. Currently, cellular telecommunications networks utilize analog control channels for the passing of control information, however, digital technology has been introduced to solve many of the limitations and problems associated with analog technology. Therefore, the cellular telecommunication industry is rapidly moving toward finalizing a standard for a Digital Control Channel (DCCH) which has total frequency agility over the entire mobile telephone frequency spectrum. The current version of the cellular industry standard for a Digital Control Channel (DCCH) is described in Project No. 3011-2 of the EIA/TIA Interim Standard IS-54-C, "Cellular System Dual-Mode Mobile Station--Base Station Compatibility Standard", dated Apr. 8, 1994, which is hereby incorporated by reference herein.
The control channel, whether analog or digital, performs the function of providing access for mobile subscribers to services provided by cells within a cellular telecommunications network. Thus, if a control channel fails, the subscribers cannot access and utilize the services provided by the cell, resulting in unsatisfied customers and decreased revenues for the network operator.
In cellular telecommunication networks, mobile calls must be processed on a continuous basis. Therefore, it is critical to have base stations operating on a continuous basis. If a control channel is implemented on a base station's transmitter, and that transmitter subsequently fails, then all mobile stations within the base station's cell are no longer able to establish new communications with the base station. Therefore, it is desirable for base stations to have a backup control channel to ensure that a high service level is maintained in the associated cell.
In existing cellular base stations and two-way trunking systems where there are multiple transmitters, a common transmit antenna may be shared among a group of radios. Proper Radio Frequency (RF) devices are required in order to couple more than one transmitter to a single antenna. RF combiners are a typical example of such a coupling device.
In low power applications (i.e. in the range of a few watts or less), hybrid type combiner technology is often used. However, for higher power (in the range of 10's to 100's of watts or more), combiner systems with combiner filters are used. Combiner filters are frequency sensitive and require tuning, performed either manually or automatically. When the tuning is performed automatically, the combiner is known as an auto-tune combiner (ATC).
A minimum frequency spacing between active channels must be implemented to ensure sufficient RF isolation, impedance matching, and good return loss (reflected RF energy) in an ATC combiner system. Proper RF isolation will not be maintained if two combiner filters are tuned to the same frequency within the combiner system.
In existing analog cellular radio systems such as the Advanced Mobile Phone Service (AMPS) in the United States, and the Total Access Communication System (TACS) in some European and Asian countries, control channels use dedicated channel numbers representing frequency pairs. When a cell experiences a control channel failure, a backup control channel operating on the same channel number normally takes over. This backup or redundant control channel may be implemented with an available voice channel transceiver in the cell. The voice channel is made available, re-configured as a control channel, and re-tuned to the correct channel number.
When manually tuned combiners are used in the cell, a coaxial switch is often used to connect the backup control channel to the combiner input of the failed control channel. The freed combiner input is then terminated. This ensures that the RF combiner system maintains its RF characteristics. The coaxial switch must be operated automatically and must have the proper interface (electrical signal) and software support.
When auto-tune combiners (ATCs) are used, no coaxial switch is needed. However, the combiner filter associated with the failed control channel must be parked. The term "parked" means re-tuned to an unused frequency so that RF isolation and other previously mentioned desirable RF characteristics are maintained. Two combiner filters tuned to the same frequency may cause impedance mismatch and/or excessive return loss. Excessive return loss (reflected RF energy) can result in permanent damage to the radio transmitter. There is currently no system or method for automatically parking the combiner filter associated with the failed control channel.
Although there are no known prior art teachings of a solution to the aforementioned deficiency and shortcoming such as that disclosed herein, a number of prior art references exist that discuss subject matter that bears some relation to matters discussed herein. Such prior art references are U.S. Pat. No. 4,829,554 to Barnes et al., and U.S. Pat. No. 5,175,866 to Childress et al. Each of these references is discussed briefly below.
U.S. Pat. No. 4,829,554 to Barnes et al. utilizes radio interface modules (RIMs) at the cell base station to separately control each communications channel at the local (base station) level. Each RIM unit independently sets up the radio portion of the link between the base station and the mobile station, and maintains and monitors the link including both communications and control signals. Barnes does not, however, teach or suggest any system or method of automatically parking ATC combiner filters in order to maintain adequate RF isolation when the backup control channel operates on the same channel number.
U.S. Pat. No. 5,175,866 to Childress et al. is a digitally trunked fail-soft architecture for a public telecommunications system. A trunking card is assigned to the control channel where it independently processes RF signals being transmitted and received by an associated repeater. During each inbound control channel slot (a time interval during which a mobile station transmits its working channel acquisition request), the trunking card determines whether or not a message is being received. If a message is being received, it is processed. If no message is received, the trunking card polls the other trunking cards in the system for status information. Although Childress permits rapid detection of control channel failure, and subsequent identification of normally operating channels, Childress does not teach or suggest any system or method of automatically parking ATC combiner filters in order to maintain adequate RF isolation when the backup control channel operates on the same channel number.
Review of each of the foregoing references, therefore, reveals no disclosure or suggestion of a system or method such as that described and claimed herein.
It would be a distinct advantage to have a system and method of automatically parking the combiner filter associated with the failed control channel in order to overcome these disadvantages. The present invention provides such a solution.